Packet switching systems transmit data by breaking the data into relatively small manageable pieces called packets. Packet switching can be used to transmit data in both computer networks and in telephone voice networks. Telephone packet switching networks transmit a series of packets over the same route in the network. Such systems in effect establish a virtual circuit from the point where a series of packets enters the network to the point where the packets are delivered. Packet switching networks establish virtual circuits through the network in order to transmit voice without delay and distortion.
Protocols such as the Internet ITPC protocol can transmit voice without establishing a virtual circuit connection, however, voice transmission using this type of protocol generally has less quality than voice transmitted using protocols which establish virtual circuits between the input point and the output point in the network.
Today, some voice transmissions are being made over packet protocols (such as the Internet) which do not establish virtual circuits. Voice connections over such circuits are of relatively low quality. The packet protocols which are used in the public telephone network are packet protocols which establish virtual circuits and which transmit all the packets that constitute a conversation over the same route through the network. Thus they provide high quality connections.
Data communication protocols can be characterized as either synchronous or asynchronous. Examples of widely used synchronous protocols are the X.25 protocol, and the frame relay protocol. Examples of widely used asynchronous protocols are the Ethernet, FDDI and ATM protocols. The X.25 protocol, the frame relay protocol and the ATM protocol are widely used in telephone systems. The Ethernet protocol and the FDDI ring protocol are widely used in local area networks (LANS) and wide area networks (WANS) that are used to interconnect computer systems.
There are various well known techniques for controlling asynchronous networks. One technique termed "carrier sense, multiple access with collision detection (CSMA/CD) is used in Ethernet networks. Another technique called token passing is used in FDDI ring networks.
Explanations of various synchronous and asynchronous protocols, and an explanation of CSMA/CD and FDDI ring networks is for example given in a book entitled "Voice and Data Communications Handbook" by Regis J. Bates and Donald Gregory which is published by McGraw Hill.